1. Field of the Invention
The present invention relates to a trailer unit especially for use with tractor trucks and, more particularly, to a multi-section trailer unit which, when empty, can self-load a rear section thereof onto a front section thereof, piggyback style.
2. Description of the Prior Art
Particularly in the hauling of logs, for instance from a logging operation to a log dump, lumber or pulp mill, open semi-trailers hitched to tractor trucks have been commonly used. These semi-trailers comprise basically an elongated flat bed for receiving the logs longitudinally thereon with uprights being provided at the sides of the semi-trailer for retaining the logs in a stacked relationship on the semi-trailer. Such semi-trailers are sometimes difficult to handle, especially when exempt of a hauling load, whereby it becomes desirable to carry the semi-trailers, when empty, on the tractor. Various self-loading and unloading systems for tractor-trailer combinations have used a cable and winch system to pull the trailer forward and upward onto the rear frame section of the tractor.
Such cable based systems did not provide adequate control of the movement of the trailer and were also relatively unsafe. Accordingly, improvements have been made to such self-loading systems, such as the loading system of U.S. Pat. No. 3,706,464 issued on Dec. 19, 1972 to Burrows et al. The loading system of this U.S. Patent is designed for loading and unloading a trailer which is releasably connected to a fifth wheel of a tractor onto the rear portion of this tractor. The tractor includes power means which are connected to the fifth wheel which itself is connected to a front end of the trailer. The loading system further comprises a lift arm and an elevatable bell crank which are operable by the power means and, more particularly, by the hydraulic cylinder thereof. More particularly, the lift arm is pivotally attached at one end thereof to the sub-frame of the tractor and at the opposite end thereof to a central section of the bell crank. The bell crank is pivotally attached at its respective ends to the fifth wheel and to the power means. Accordingly, the double-acting power cylinder interconnects the tractor frame and the bell crank. When actuated, the cylinder raises the lift arm, the bell crank and the fifth wheel forward and upward over the rear frame of the tractor thereby causing the empty trailer coupled to the fifth wheel to be pulled onto the tractor until the trailer wheels are supported on the rear frame of the tractor. Accordingly, in the system disclosed in U.S. Pat. No. 3,706,464, it is necessary to substantially modify the rear of the tractor and, more particularly, a new fifth wheel assembly is required and a drive system for displacing the fifth wheel must be added to the tractor, this loading system including mainly a double-acting hydraulic cylinder, a lift arm and a bell crank. Furthermore, a rearward most end of the tractor must be provided with a ramp and side brackets for supporting and guiding the trailer wheels during the loading and unloading of the trailer onto or from the tractor truck.
Similarly, Canadian Patent No. 977,000 issued on Oct. 28, 1975 to Isley also discloses a log hauling equipment in which the tractor truck is provided with trailer loading means, the trailer including a longitudinally oriented telescopic main member and trailer wheels provided at a rear end of the main member. The front end of the main member is coupled to the loading mechanism provided at a rear end of the tractor truck. The loading system comprises a lift arm pivotable in a vertical plane by way of hydraulic cylinders between a ground position wherein the lift arm or coupling bar extends substantially horizontally rearwards of the tractor and is basically collinear with the main member of the trailer, and a carry position wherein the lift arm extends forwardly from its pivot point and substantially opposite its position in the ground position of the trailer. Accordingly, actuation of the hydraulic cylinders causes the lift arm to pivot upwards and forwards toward the tractor cab thereby causing the trailer which has its main member in a telescopically collapsed position to climb on ramps provided at a rearward most end of the tractor truck until the trailer wheels rest on the rear portion of the tractor truck.
As in the aforementioned U.S. Patent, Canadian Patent No. 977,000 necessitates substantial modifications to the tractor. Furthermore, in both systems, the center of gravity of the trailer, in the carry position thereof onto the tractor, is located rearwards of the conventional location of the fifth wheel of the tractor above and longitudinally between the double-axelled rear wheels of the tractor. In other words, the weight of the trailer, in the carry position, is not ideally centered over the rear wheels of the tractor. Accordingly, the tractor can become substantially thrown off balance.